Simple Interest Calculator: Days & Year Basis
Calculate simple interest from a principal amount, a fixed annual rate and a number of days. Choose a fixed year basis of 365, 360 or 366 days. Interest is calculated on the original principal only; it does not compound.
How to calculate simple interest by days
Simple interest = principal x (annual interest rate / 100) x (number of days / year basis). Principal plus interest = original principal + simple interest. The year basis sets how the entered number of days is expressed as a fraction of a year.
The calculator retains the exact decimal inputs for its interest calculation, then rounds the final interest amount half up to the nearest cent. It does not round a daily charge first. The result assumes the principal and rate stay constant throughout the entered period.
Worked example: 90 days at 6 percent
Enter Principal amount 10000, Annual interest rate (%) 6 and Number of days 90. Leave Year basis at 365 days (fixed), then select Calculate simple interest. The unrounded interest is 10000 x 0.06 x 90 / 365 = 147.945205..., giving Simple interest 147.95 and Principal plus interest 10147.95.
Change Year basis to 360 days (fixed) and calculate again. Interest becomes 10000 x 0.06 x 90 / 360 = 150.00, and principal plus interest becomes 10150.00. The larger amount comes from the smaller denominator, not from a different annual rate.
Choose the day-count basis carefully
A fixed 365-day basis divides all entered days by 365. A fixed 360-day basis divides them by 360. A fixed 366-day basis divides them by 366. Use the convention in the relevant agreement rather than choosing whichever result is preferable.
Selecting 360 does not automatically turn calendar dates into 30-day months. A 30/360 convention adjusts a date-based numerator as well as using a 360-day denominator, and different 30/360 variants have different rules. This tool only uses the day count you enter.
Likewise, selecting 366 does not automatically implement Actual/Actual. Actual/Actual conventions can split a period across calendar years and apply different denominators. This calculator uses one fixed denominator for the entire period, even if the entered days cross a year boundary.
Simple interest is different from compound interest
With simple interest, previously accrued interest does not earn additional interest. Doubling the days doubles unrounded interest when the principal, rate and basis stay constant. Compounding adds interest to a balance at defined intervals and produces a different calculation.
For growth that compounds, use the Compound Interest Calculator. For monthly interest payments with a principal balloon at the end, use the Interest-Only Loan Calculator, which has a different rounding and payment model.
Limits and excluded charges
Principal must be greater than zero up to 1000000000, with at most two decimal places. The annual rate may be 0 to 100 percent, with at most four decimal places. Enter 0 to 100000 whole days. Zero days or a zero rate gives zero interest and leaves principal unchanged.
The model excludes fees, taxes, repayments during the period, variable rates, daily rounding and compounding. It does not calculate a repayment schedule, effective annual yield or APR including charges. All amounts assume a currency with two decimal places.
For a step-by-step walkthrough, read How to Calculate Simple Interest by Days: 365 vs 360.
Privacy
Calculations run locally in your browser. This calculator does not save or send the entered amounts. The hosting website may use separate analytics or advertising services.
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